Flame detection apparatus



March 17,

E. JELLINEK FLAME DETECTION APPARATUS Filed NOV. 19, 1949 VOLTAGE 4WD? I9 POTENTIAE A 4 union: :0 POTENflAL A L V B NOFLAME FLAME l GROUNbE D PROBE Inventor: Ernesc JeHmek, by WM is Attorney.

Patented Mar. 17, 1953 UNITED STATES FLAME DETECTION APPARATUS Ernest Jellinek, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application November 19, 1949, Serial No. 128,443

15 Claims.

My invention relates to electronic flame detection apparatus, and is particularly useful in the automatic control of gas or oil burning apparatus.

In gas and oil burning systems Where a main or pilot gas flame is ignited by an electric spark, it has been found that a probe electrode positioned in the flame may pick up an electrostatic voltage not in any way related to a control voltage caused by conduction through the flame. Such electrostatic voltage has been found to produce false operation in some flame-responsive devices wherein an electron discharge device is controlled merely by the static voltage between the probe electrode and ground. It is also desirable, in such systems, to prevent false operation by grounding of the probe electrode, either directly or through an accumulation of soot between it and ground.

Accordingly, it is a general object of my inven tion to provide a new and novel flame detection apparatus.

It is a more particular object of my invention to provide an electronic flame detection apparatus having a flame probe and which is insensitive to externally impressed negative voltages which may be imposed upon the probe independently of the control voltage.

Still another object of my invention is to provide an electronic flame detector which is insensitive to shunt capacitive impedance in the wire connected between the control panel and the flame-responsive element.

Another object of the invention is to provide new and novel means for indicating grounding of the flame probe, or other short circuiting of the conducting gap.

A still further object of the invention is to provide new and novel means for indicating the presence of a soot accumulation between the flame electrodes, which would otherwise result in a false indication of the presence of a flame.

My invention itself will be further understood and. its various objects and advantages more fully appreciated by referring now to the following detailed specification taken in conjunction with the accompanying drawing, in which Fig. 1 is a schematic circuit diagram of a flame detection apparatus embodying my invention; Fig. 2 is a fragmentary schematic circuit diagram of a portion of the same apparatus showing the flame electrode circuit in elementary form; and Fig. 3 is a graphical representation of the plate and grid potentials supplied to the detection tube under various operating conditions.

Referring now to the drawing, and more particularly to Fig. 1, I have shown, by way of illustration of one embodiment of my invention, a flame detection apparatus suitable for use in conjunction with an oil burning apparatus of the commercial type having a main oil supply nozzle i, a pilot gas nozzle 2 positioned to ignite the oil,

and pair of s ark ignition electrodes 3 energized through an ignition transformer 4 from a suitable source of alternating current supply (not shown) and positioned to ignite the pilot gas. Any suitable control system may be utilized for initiating operation of the oil burning apparatus by first energizing the spark electrodes 3 and admitting gas to the nozzle 2 to be ignited by the spark, and then admitting oil to the nozzle I to be ignited by the gas flame.

In order to detect whether or not a pilot gas flame is present at the nozzle 2, I provide a flame detection apparatus including a pair of electron discharge devices 6 and I and an electroresponsive control device, such as a relay having an exciting Winding 5 shunted by a holding capacitor 5a. The relay may suitably be connected to control a valve (not shown) in the fuel supply line for the main burner nozzle l. The relay winding .5 is connected in the anode circuit of the discharge device 6, so that it is energized whenever the discharge device 6 is conductive. The discharge device 6 is connected as an amplifier under the control of the discharge device 1, which serves as a detector of the presence of flame. The amplifier 6 is a three-element discharge tube having an anode 8, a cathode 9 and a control electrode Hi. The output or anode circuit of the amplifier tube 6 is supplied with alternating current from opposite ends of a secondary winding H on a supply transformer I2. The supply transformer i2 is provided with a primary winding 53, and an intermediate point of the secondary winding H is grounded through a current limiting resistor I i.

The control electrode Iii of the amplifier tube 6 is connected to the amplifier cathode 9 through an input circuit including a current limiting resistor l5 and a pair of biasing resistors l5 and Il, all in series circuit relation. The biasing resrstor I6 is shunted by a capacitor Ilia and the biasing resistor I1 is shunted by a capacitor 1 7a,. The grid biasing resistor I! and its parallel-corrnected capacitor Ila are connected across a portion of the transformer secondary winding it through a unilateral conducting device or rectifier [8. The biasing resistor H has one end connected to that end of the secondary winding H to which the amplifier cathode 9 is connected and has its other end connected through the rec tlfier 8 to a point on the winding i I intermediate the amplifier cathode connection and the grounded point of the winding. The rectifier I8 is so disposed that the charge maintained on the capacitor 21a, by rectification through the rectifier it has a polarity tending to bias the amplifier grid H] positive with respect to the cathode 9 and thus render the amplifier 6 conductive.

The gr d bias resistor It in the input circuit of the amplifier 6 is connected also in the output or anode c1rcuit of the detector tube 7 so that when the detector tube 1 is conductive, the capacitor Isa is charged. The capacitor I6a is so connected in the amplifier input circuit that its charge opposes that maintained on the capacitor IIa and tends to place a negative bias potential upon the grid I9 of the amplifier G. The D. C. potential across the capacitor I611 and resistor I5, when the detector I is conducting, is greater than the opposing D. C. potential across the capacitor Ila and resistor !'I, so that when the detector tube 1 is conducting, the net grid bias potential upon the amplifier tube is negative, and the amplifier is non-conductive.

The detector tube '1 is a three-element electron discharge device having an anode IS, a cathode and a control electrode H. The anode-cathode circuit of the detector tube I is supplied with alternating potential from a tertiary Winding 23 on the supply transformer I2, and includes in series circuit relation the amplifier grid bias resistor I6 and a current limiting resistor 24.

The control electrode 2I of the detector tube I is connected to the cathode 2% through a current limiting resistor 25 and a grid bias resistor 25 in series circuit relation. The grid resistor 26 is shunted by a relatively small resistor 21 and a relatively large capacitor 28 in series. The grid resistor 25 is connected also in a probe or flame discharge circuit including a flame responsive impedance and the resistor 25 in series. The probe circuit is so arranged that, under certain conditions of operation, flow of current therethrough impresses across the resistor 25 various potential differences efiective to bias the detector tube '5 to various desired conditions or" operation. The probe circuit is shown in simplified schematic form at Fig. 2, wherein it will be understood that the anode It and cathode 29 of the detector tube I are connected in the manner illustrated at Fig. 1.

Referring more particularly now to Fig. 2, the detector grid is connected through the current limiting resistor 25 and a shielded cable 29 to a probe electrode 36, which is positioned near the grounded gas burner nozzle 2. The grid end of the bias resistor 26 is thus connected to the probe 30 through the cable 29. The burner nozzle 2 and the probe electrode 30 serve as a pair of spaced-apart flame electrodes defining between them an ionizable discharge gap adapted to be bridged by a flame. In parallel with the flame gap, there is connected a photoelectric discharge device 3I in series with a current-limiting resistor 3Ia, positioned to view the main oil flame. The conducting cable sheath 29a is connected to the cathode end of the grid resistor 25 (and thus to the detector cathode 2i!) and is connected also to a conducting guard ring 32 surrounding the probe electrode 38 adjacent its tip. The purpose of this guard ring connection will be more fully described hereinafter.

The cathode end of the grid resistor 26 is connected to ground through series connected sources of alternating and unidirectional potential. The source of unidirectional potential for the probe circuit is constituted by the capacitor I'Ia, which is maintained charged, as previously described, through a rectifier I8 from a portion of the transformer secondary Winding I I. The source of alternating potential for the probe circuit is constituted by that portion of the transformer secondary winding I I between the amplifier cathode end of the winding and the grounded point. This portion of the transformer secondary winding is designated at Fig. 2 as Ila, and provides an alternating voltage component in the probe circuit which is substantially in phase with the alternating potential of the transformer tertiary winding 23. This is indicated by the polarity dots adjacent one end of each of transformer windings II and 23; a dot indicates the positive voltage end of each of these windings when a positively increasing current is entering the dotted end of a winding. The series connected alternating and direct potential sources in the probe circuit are such that under open circuit, or no flame, conditions, the peak magnitude of the alternating potential preferably is at least as great or greater than the direct potential.

The probe circuit then may be traced from ground through the current limiting resistor I4, the alternating current potential source IIa, the unidirectional potential source I'Ia, the grid resistor 26 and the flame electrodes 30 and 2 to ground. In this circuit, the capacitor Ila is so disposed that when the flame gap is conducting, unidirectional current flows through the grid resister 26 from the cathode end toward the grid end. The ionizable gap bridged by the flame is known to possess unidirectional conducting characteristics, and its resistance is minimum to current flow in this direction. The parallel connected photoelectric cell (H is disposed with its cathode 3Ib connected to ground and its anode 3Ic connected to the conductor 29, so that unidirectional current can flow also through the photo-tube from the conductor 29 to ground.

In the operation of my new and novel flame detection circuit connected in the manner described above, it will be evident that when no flame exists at either the main burner I or the pilot burner 2, no current flows through the probe circuit, because both the flame gap and the phototube 3I are non-conductive. With no current in the probe circuit, no external biasing potential is impressed between the grid and the cathode of the detector tube I. Therefore, since anode potential is impressed upon the detector tube I from the winding 23, the detector tube is conductive and current is passed through the detector anode circuit including the resistor I6, thereby to charge the capacitor Ifia with the polarity indicated at Fig. 1. This charge on the capacitor I50. is greater in magnitude than the opposing charge maintained on the capacitor I'ia, so that the net biasing potential applied to the amplifier grid I0 is negative and the amplifier tube 6 is maintained non-conductive.

The foregoing condition is illustrated at the left hand portion of Fig. 3, where curve A represents the alternating anode voltage on the detector tube 7 and the line B represents the slight negative voltage drop across the grid resistor 26 due to grid rectification.

When a gas flame is ignited at the burner nozzle 2, the flame bridges the gap between the nozzle and, the probe electrode 39 and completes an electric conducting path through the probe circuit illustrated at Fig. 2. The constants of this circuit and the direct and alternating potential values are such that with normal flame resistance, the unidirectional current component is predominant and the alternating current component is relatively small. Thus, an undulating direct current traverses the probe circuit and passes through the grid resistor 26 from the cathode end toward the grid end. This probe circuit current impresses across the resistor 26 a voltage drop illustrated by the curve C of Fig. 3, and renders the control electrode 2i of the detector I negative with respect to the cathode 2 B. The negative bias potential thus applied to the detector tube 1 as a result of flame current renders the detector 1 non-conductive. When the detector becomes non-conductive, the charge on the capacitor led in the detector anode circuit is abruptly reduced, and the amplifier grid is eiiectively biased positive with respect to the cathode by the charge on the capacitor i'la. The amplifier tube 5 then becomes conductive, thereby to energize the flame detector relay winding 5.

In the event that the probe electrode 39 comes in contact with the gas burner nozzle 2 or is otherwise grounded, thereby to short circuit the flame, the resistance of the probe circuit current path is so reduced that both the unidirectional and alternating components of current from the sources lid and Ma, respectively, are increased in magnitude. As previously stated, the peak alternating potential component preferably is as great as or greater than the direct potential component. The increase in magnitude of the unidirectional. ourrent component is relatively small, because of the relatively large value of the grid resistor 25. That is, most or the unidirectional potential drop occurring in the probe circuit normally occurs across resistor 26 and, therefore, when the flame electrodes are shorted only a relatively small additional potential drop is available to be added to t. e potential drop already present across this resistor. The alternating current component, however, increases in magnitude sufflciently to become predominant, by reason of the relatively small value of the resistor 2'! and the large capacitance of the capacitor 28. In the case of the alternating potential drop only a relatively small portion of the drop normally takes place across resistor 27 and capacitor 28 and, therefore, when the flame electrodes are shorted, a considerable additional drop is available for addition to the normal drop. Under the stated condition, i. e., with probe electrode 3b grounded, the resultant potential drop across the grid resistor 25 is an alternating potential demonstrating periodic reversals of direction, so that on alternate half cycles, the potential drop across the grid resistor 26 is such as to bias the detector grid 2! positive (or at least sufiiciently less negative that the grid is above cutofl potential) with respect to the cathode 2b. This condition is illustrated at the right side of Fig. 3, wherein the line D represents the potential drop across the grid resistor 2t from cathode to grid. Alternating current in the probe circuit is permitted under this condition because the rectifying flame impedance is no longer present in the circuit. Since the alternating current component in the probe circuit is substantially in phase with the detector anode potential supplied by the tertiary transformer winding 23, the grid 2| is positive for a portion of each half cycle during which the anode is positive, so that the detector tube 1 again becomes conductive. When the detector tube becomes conductive, the capacitor llia is again charged in a direction to .bias the amplifier tube 6 to cutoff. Thus, grounding of the probe electrode 353 causes deenergization of the flame detector relay winding 5.

In the event that an accumulation of soot on the probe electrode (it becomes so great as to connect the probe electrode electrically with the conducting guard ring 32, a short circuit is established around the grid resistor 26 through the conductor 2c, the probe electrode til, the guard ring 32 and the conductor shield 29a. By the establishment of such a short circuit, the resistor 26 is prevented from imposing any biasing effect upon the detector grid 2|, so that the detector tube, being unbiased, again becomes conductive, thereby to render the amplifier tube 6 non-conductive and to deenergize the flame detector relay winding 5.

In normal operation, where a flame at the gas pilot burner 2 is properly established and the amplifier tube 5 rendered conductive as described, and where a flame at the oil discharge nozzle i is subsequently ignited, the probe circuit current is maintained through the photoelectric tube 3!, and the gas pilot flame may be interrupted.

It will now be evident that in accordance with my invention, I have provided a new and novel flame detection apparatus of the electronic type including an amplifier tube and a detector tube wherein the input circuit of the detector tube is controlled solely in accordance with actual control current flowing through a flame-responsive gap. Under normal flame conditions, this current is an undulating unidirectional current and is not appreciably affected by capacitance of the cable interconnecting the flame controlled gap and the detector tube. Nor do flame variations creating impedance variations in the circuit afiect the apparatus adversely inasmuch as the combination of the capacitor 28 and resistor 2% provides an arrangement having a large time constant for current changes in the probe circuit. Moreover, since for the purpose of indicating grounding of the flame electrode the device is dependent for operation upon actual conduction through the probe circuit, it is insensitive to externally imposed negative probe potentials not related to the control potential in the probe circuit.

While I have described a preferred embodiment of my invention by way of illustration, many modifications will occur to those skilled in the art and I, therefore, wish to have it understood that I intend in the appended claims to cover all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a flame detection apparatus, a first electron discharge device having input and output circuits, a control device connected in said output circuit, a second electron discharge device having an anode, a cathode, and a control electrode, a source of alternating potential connected to said anode, a circuit connecting said anode and cathode to control said input circuit of said first discharge device, said circuit including said source of potential and maintaining said first discharge device non-conducting when said second discharge device is conducting, a large resistor connected between said cathode and said control electrode, whereby said second discharge device is normally conducting, a large capacitor and a small resistor serially connected in shunt relation with said large resistor, a pair of flame electrodes adapted to be positioned in a flame and defining an ioniaable gap bridged by said flame, a series circuit including said large resistor and said gap, means for impressing upon said series circuit a potential having a unidirectional component and an alternating component and adapted in the presence of flame to supply to said large resistor an undulating unidirectional current in a direction to render said second discharge device non-conducting, said alternating potential component being substantially in phase with said anode potential, said capacitor and said small resistor providing a low 7 impedance path for said alternating current component whereby short circuiting of said gap impresses across said large resistor an alternating potential adapted to render said second discharge device conducting.

2. In a flame detection apparatus, a first electron discharge device having input and output circuits, a control device in said output circuit, a second electron discharge device having an anode, a cathode and a control electrode, means connecting said anode and cathode to control said input circuit of said first discharge device, a pair of impedances connected in parallel between said cathode and said control electrode, a first one of said impedances having a relatively high value of impedance and being suitable for carrying both unidirectional and alternating current, the second of said impedances having a relatively low value of impedance and being suitable for carrying alternating current but not unidirectional current, a pair of flame electrodes adapted to be positioned in a flame and defining an ionizable gap bridged by said flame, connections to a source of unidirectional potential, connections to a source of alternating potential, a flameresponsive circuit including in series circuit relation said sources of potential, said pair of impedances and said flame electrodes, a guard ring positioned around at least one of said flame electrodes and adapted to be electrically connected thereto through an accumulation of soot on said one flame electrode, and means connecting said guard ring and said one flame electrode to short circuit said pair of impedances upon the occurrence of such soot accumulation.

3. In a flame detection apparatus, a first electron discharge device having input and output circuits, a control device connected in said output circuit, a second electron discharge device having an anode, a cathode, and a control electrode, means connecting said anode and said cathode to control said input circuit of said first discharge device, said connecting means rendering said first discharge device conductive when said second discharge device is non-conductive, a first resistor having a relatively large value of impedance connected between said cathode and said control electrode, a second resistor having a relatively low value of impedance and a serially connected capacitor having a relatively low value of impedance connected in shunt with said first resistor, a pair of flame electrodes including a conducting probe adapted to be positioned in a flame, said flame electrodes defining an ionizable gap bridged by said flame, a source of unidirectional potential, a source of alternating potential, and a series-connected flame-responsive circuit including said sources, said resistors and capacitor and said flame electrodes, said series circuit being adapted in the presence of flame to impress across said first resistor a pulsating unidirectional potential having a polarity arranged to bias said second discharge device to a condition of non-conduction, a guard electrode positioned adjacent said probe and spaced therefrom by a gap adapted to be bridged by an accumulation of soot on said probe, and means connecting said guard electrode to shunt said first resistor and said second resistor and capacitor upon the occurrence of such soot accumulation, thereby to render said second discharge device conductive.

4. In a flame detection apparatus, an electron discharge device having an anode, a cathode and a control electrode, a pair of impedances connected in parallel between said cathode and said control electrode, connections for connecting said pair of impedances in a series circuit with a pair of spaced electrodes insulated from each other and adapted to be positioned in the path of a flame, a first one of said impedances having a relatively high value of impedance and being suitable for carrying both unidirectional and alternating current, the second of said impedances having a relatively low value of impedance and being suitable for carrying alternating current but not unidirectional current, connections to a source of unidirectional potential for energizing said series circuit, and connections to a source of alternating potential for energizing said series circuit simultaneously with the unidirectional potential, the peak voltage of the alternating potential being at least substantially as great as the voltage Or the unidirectional potential.

5. In a flame detection apparatus, an electron discharge device having an anode, a cathode and a control electrode, means connecting said anode and said cathode to a source of alternating potential, a resistor connecting said cathode and said control electrode, connections for serially connecting said resistor in circuit with spaced electrodes insulated from each other and adapted to be positioned in the path of a flame, connections to a source of unidirectional potential for supplying through said resistor in the presence of flame a unidirectional current in the direction to bias said control electrode negative with respect to said cathode and render said discharge device non-conductive, an impedance suitable for carrying alternating current but not unidirectional current connected in shuntwith said rcsistor, and connections to a source of alternating potential for supplying through said shunt impedance an alternatin current substantially in phase with said first-mentioned alternating potential when the resistance between said flame electrodes is substantially zero whereby said electron discharge device is rendered conductive when said flame path is short-circuited.

6. In a flame detection apparatus, a first electron discharge device having input and output circuits, a control device in said output circuit, a second electron discharge device having an anode, a cathode, and a control electrode, means connecting said anode and cathode to control said input circuit of said first discharge device, a resistor connected between said cathode and said control electrode, a flame responsive circuit including said resistor and an ionizable gap adapted to be bridged by a flame, connections to a source of unidirectional potential for energizing said flame responsive circuit, said second discharge device being conductive in the absence of flame and said connections to said unidirectional source being so arranged that current through said resistor and gap in the presence oi flame renders said control electrode negative with respect to said cathode and terminates conduction in said second discharge device, an impedance connected in shunt with said resistor, said impedance being suitable for carrying alternating current but not unidirectional current and having a value of impedance which is relatively small compared t the impedance of said resistor, and connections to a source of alternating potential for energizing said flame responsive circuit simultaneously with the unidirectional potential, the peak voltage of the alternating potential being substantially at least as great as the 9 voltage of the unidirectional potential whereb when said ionizable gap becomes short-circuited the alternating potential drop across said resistor and said impedance overcomes the unidirectional potential drop across said resistor and renders said control electrode sufliciently less negative with respect to said cathode that the said second discharge device again conducts.

7. In a flame detection apparatus, a first electron discharge device having input and output circuits, a control device in said output circuit, a second electron discharge device having an anode, a cathode, and a control electrode, means connecting said anode and cathode to control said input circuit of said first discharge device, a pair of impedances connected in parallel between said cathode and said control electrode, a first one of said impedances having a relatively high value of impedance and being suitable for carrying both unidirectional and alternating current, the second of said impedances having a relatively low value of impedance and being suitable for carrying alternating current but not unidirectional current, a source of alternating potential and a source of unidirectional potential connected in series circuit relation, and a flame responsive circuit including said impedances and an ionizable gap adapted to be bridged by a flame connected in series circuit relation across said series-connected potential sources.

8. In a flame detection apparatus, a first electron discharge device having input and output circuits, a control device in said output circuit, a second electron discharge device having an anode, a cathode, and a control electrode, means connecting said anode and cathode to control Said input circuit of said first discharge device, a resistor connected between said cathode and said control electrode, a pair of spaced-apart flame electrodes defining an ionizable conducting gap adapted to be bridged by a flame and connected in series circuit relation with said resistor, means for supplying through said resistor and across said gap in the presence of flame a pulsating unidirectional current having an alternating current component and a direct current component, and means including an impedance connected in shunt with said resistor and suitable for carrying alternating current but not unidirectional current for increasing said alternating current component sufliciently in magnitude upon the short-circuiting of said gap substantially to overcome said direct current component.

9. In a flame detection apparatus, a first electron discharge device having input and output circuits, a control device in said output circuit, 'a second electron discharge device having an anode, a cathode and a control electrode, a first source of alternating potential, means connecting said source of potential and said anode and cathode to control said input circuit of said first discharge device, a resistor connected between said control electrode and said cathode, a pair of spaced-apart flame electrodes defining an ionizable conducting gap adapted to be bridged by a flame and connected in a series circuit with said resistor, connections to a source of unidirectional potential for energizing said series circuit, connections to a second source of alternating potential for energizin said series circuit simultaneously with the unidirectional potential, the unidirectional potential source and the second alternating potential source producing a pulsating unidirectional current in said resistor having a unidirectional current component and an alternating current component substantially in phase with said first alternating potential, and an impedance including a second resistor in series with a capacitor connected in shunt with said first resistor, said impedance having a relatively low value of impedance compared to the impedance of said first resistor whereby the short-circuiting of said gap renders said alternating current component sufficiently greater in magnitude than said unidirectional component to periodically reverse the current through said resistor.

10. In a flame detection apparatus, an electron discharge device having an anode, a cathode, and a control electrode, means connecting said cathode and said anode to a source of alternating anode potential, a resistor connected between said control electrode and said cathode, a pair of spaced apart flame electrodes defining an ionizable gap adapted to be bridged by a flame and connected in series circuit relation with said resistor, means supplying through said resistor and across said gap in the presence of flame a pulsating unidirectional current having an alternating current component substantially in phase with the alternating anode potential, said unidirectional current being in the direction to apply through said resistor a negative cutofl biasing potential to said discharge device, and means including an impedance suitable for carrying alternating current but not unidirectional current shunting said resistor for rendering said alternating current component predominant upon the short-circuiting of said gap, thereby to apply to said discharge device an alternating in-phase biasing potential rendering said discharge device conductive.

11. In a flame detection apparatus, a first electron discharge device having input and output circuits, a control device connected in said output circuit, a second electron discharge device having an anode, a cathode and a control elec trode, a source of alternating anode potential, a circuit connecting said source of alternating potential and said anode and cathode to control said input circuit of said first discharge device, said circuit maintaining said first discharge device non-conductive when said second discharge device is conductive, a first resistor connected between said cathode and said control electrode whereby said second discharge device is normally conductive, a second resistor and a serially connected capacitor connected in shunt with said resistor, the impedance of said second resistor and serially connected capacitor being substantially less than the impedance of said first resistor, a pair of flame electrodes adapted to be positioned in a flame and defining an ionizable gap bridged by said flame, a series circuit including said gap and the parallel combination of said first resistor and said second resistor and capacitor, means for impressing upon said series circuit a unidirectional potential adapted in the presence of a predetermined range of flame resistance to render said second discharge device non-conductive, and means for impressing upon said series circuit an alternating potential in phase with said anode potential and adapted upon short circuiting of said gap to overcome the effect of said unidirectional potential and render said second discharge device conductive.

12. A control system for an electron discharge device having an anode, a cathode and a control electrode, comprising a pair of impedances connected in parallel between the cathode and the control electrode of the electron discharge device, a first one of said impedances having a relatively high value of impedance and being suitable for carrying both unidirectional and alternating current, the second of said impedances having a relatively low value of impedance and being suitable for carrying alternating current but not unidirectional current, connections for connecting said first and second impedances in series circuit with a third variable impedance, connections to a source of unidirectional potential for energizing said series circuit, and connections to a source of alternating potential for energizing said series circuit simultaneously with the unidirectional potential, the peak voltage of the alternating potential being substantially at least as great as the voltage of the unidirectional potential, whereby when said third variable impedance is at a relatively high value the voltage drop across said first impedance predominates in producing a biasing potential between the cathode and the control electrode, while when said third variable impedance is at a relatively low value the voltage drop across said second impedance predominates in producing a biasing potential between the cathode and the control electrode.

13. A control system for an electron discharge device having an anode, a cathode and a control electrode, comprising a first source of alternating potential connected between the anode and cathode, a pair of impedances connected in parallel between the cathode and the control electrode, a first one of said impedances having a relatively high value of impedance and being suitable for carrying both unidirectional and alternating current, the second of said impedances having a relatively low value impedance and being suitable for carrying alternating current but not undirectional current, a third impedance connected in a series circuit with said first and second impedances, connections to a source of unidirectional potential for energizing said series circuit, and connections to a second source of alternating potential substantially in phase with said first source of alternating potential for energizing said series circuit simultaneously with the said unidirectional potential, the peak voltage of the second alternating potential source being substantially at least as great as the voltage of the unidirectional potential, whereby when said third impedance is in circuit with the first and second impedances the voltage drop across said first impedance predominates in producing a biasing potential between the cathode and the control electrode, while when said third variable impedance is short-circuited the voltage drop across said second impedance predominates in producing a biasing potential between the cathode and the control electrode.

14. A control system for an electron discharge device having an anode, a cathode and a control electrode, comprising a pair of impedances connected in parallel between the cathode and the control electrode of the electron discharge device, a first one of said impedances having a relatively high value of impedance and being suitable for carrying both unidirectional and alternating current, the second of said impedances having a relatively low value of impedance and being suitable for carrying alternating current but not unidirectional current, a third impedance connected in a series circuit with said first and second impedances, connections to a source of unidirectional potential for energizing said series circuit in a direction such that said control electrode is negative with respect to said cathode, and connections to a source of alternating current for energizing said series circuit simultaneously with the unidirectional potential, the peak voltage of the alternating potential being substantially at least as great as the voltage of the unidirectional potential, whereby when said third impedance is in the said series circuit the voltage drop across said first impedance predominates in producing a pulsating unidirectional biasing potential between the cathode and the control electrode of suflicient magnitude that the electron discharge device is biased beyond cutoff, while when said third impedance is shortcircuited the voltage drop across said second impedance predominates and the biasing potential periodically becomes sufficiently less negative that the control electrode potential is periodically above the cutoff value.

15. A control system for an electron discharge device having an anode, a cathode and a control electrode, comprising a first source of alternating potential connected to said anode and said cathode, a first resistor having a relatively large value of resistance connected between the cathode and the control electrode of the said electron discharge device, a resistor of relatively small resistance and a serially connected capacitor of relatively large capacitance connected in parallel with said first resistor, a variable impedance connected in a series circuit with first resistor, and said second resistor and capacitor, connections to a source of unidirectional potential for energizing said series circuit in a direction such that said control electrode is negative with respect to said cathode and to an extent such that said electron discharge device is normally biased beyond cutoff, and connections to a second source of alternating potential substantially in phase with said first source of alternating potential for energizing said series circuit simultaneously with the unidirectional potential, the peak voltage of the said second alternating potential being at least as great as the voltage of the unidirectional potential, whereby when said variable impedance is at a relatively high value the voltage drop across said first resistor predominates and a pulsating unidirectional biasing potential results between the cathode and the control electrode, while when said variable impedance is at a relatively low value the voltage drop across the said second resistor and the capacitor predominates and produces an alternating biasing potential between the cathode and the control electrode which is maximum during positive half cycles of said first alternating current potential and causes said electron discharge device to become conducting during such half cycles.

ERNEST JELLINEK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,162,501 Draper June 13, 1939 2,224,119 Harrison Dec. 3, 19 10 2,231,420 Gille Feb. 11, 1941 2,352,143 Wills June 20, 1944 2,383,806 Kubler et al Aug. 28, 1945 2,455,351 Beam et al Dec. 7, 1948 

